1. Field of the Invention
The present invention relates to a fixing device that fixes a toner image on a recording medium and also relates to an image forming apparatus including the fixing device. Examples of the image forming apparatus include a copying machine, a printer, a facsimile, and a multifunction peripheral (MFP) having these functions.
2. Description of the Related Art
In recent years, energy saving and speeding-up have been commercially required increasingly for an image forming apparatus such as a copying machine. In order to achieve these requirements, it is important to improve the thermal efficiency of a fixing device used in the image forming apparatus.
In the image forming apparatus, a non-fixed toner image is formed on a recording medium such as a recording material sheet, printing paper, photosensitive paper, or electrostatic recording paper in accordance with an image transfer system or a direct system by an image forming process of electrophotographic recording, electrostatic recording, magnetic recording or the like. Fixing devices of contact heating systems are widely employed as fixing devices for fixing a non-fixed toner image. Examples of the contact heating systems include a heat roller system, a film heating system, and an electromagnetic inductive heating system.
A fixing device of a heat roller system basically includes a pair of rotating rollers including a fixing roller and a pressing roller. The fixing roller has a heat source such as a halogen lamp therein, so that the temperature of the fixing roller is adjusted to a predetermined temperature. The pressing roller makes pressure-contact with the fixing roller. A recording medium is guided to a so-called fixing nip portion as a contact portion between the pair of the rotating rollers so as to be conveyed. Then, a non-fixed toner image is fused and fixed by heat and pressure from the fixing roller and the pressing roller.
A fixing device of a heat roller system having the following configuration is disclosed in Japanese Patent Application Laid-open No. 2008-065042. In this fixing device, even when a member of a separating section is configured to be detachably connected or depressurized in order to enhance usage life of a fixing rotating member, a tip of the separating member can be responded to fluctuation on a surface of the fixing rotating member on a nip portion.
Furthermore, a fixing device of a film heating system is disclosed in Japanese Patent Application Laid-open No. S63-313182 and Japanese Patent Application Laid-open No. H1-263679. The fixing device makes a recording medium close contact with a heating body that is fixedly supported by a supporting member through a thin fixing film having heat resistance, so that heat of the heating body is supplied to the recording medium through the film material while the fixing film is slidingly moved on the heating body. In the fixing device, a ceramic heater including a resistance layer on a ceramic substrate such as alumina, or aluminum nitride is used as the heating body, for example. Such ceramic substrate has properties of heat resistance, insulation property, excellent heat conductivity, and the like. In the fixing device, a thin film having low heat capacity can be used as the fixing film. Therefore, the fixing device has higher heat transfer efficiency than that of the fixing device of the heat roller system. Accordingly, a warm-up time can be reduced and quick start and energy saving can be realized in the fixing device.
As a fixing device of an electromagnetic inductive heating system, the following fixing device is disclosed in Japanese Utility Model Laid-open No. S51-109739. In this fixing device, an eddy current is induced in a metal layer (heat generating layer) of a fixing sleeve with magnetic flux so as to generate heat with Joule heat thereof. With the fixing device, a fixing film itself produces heat by utilizing generation of induction current, so that a fixing process having higher efficiency than that in the fixing device of the heat roller system in which a halogen lamp is used as a heat source can be realized.
As a fixing device of an electromagnetic inductive heating system, the following configuration has been well known. The fixing device includes a fixing sleeve having a release layer, an elastic layer, a metal layer (heat generating layer), and a fixing roller that is formed by the elastic layer included in the fixing sleeve and a supporting body (core). Furthermore, in the fixing device, the fixing roller and a pressing roller are made pressure-contact with each other through the fixing sleeve so as to form a pressure-contact nip portion.
In the configuration, the fixing sleeve is prevented from moving (displacing) in the thrust direction by adhering the fixing sleeve and the fixing roller to each other with a silicone adhesive or the like. Alternatively, the fixing sleeve is prevented from moving (advancing) in the thrust direction by arranging a ring having a diameter larger than that of the fixing sleeve on an end of the fixing roller when the fixing sleeve and the fixing roller are not adhered to each other.
A recording medium onto which a toner image has been fixed on the fixing nip portion formed by the fixing roller and the pressing roller is discharged in the direction of winding up around the fixing roller or the pressing roller depending on viscosity of toner that is molten and not cooled or the orientation of the fixing nip portion. Therefore, a separating member has been conventionally used for guiding the recording medium to a right conveying path. A contact type claw has been used as the separating member in the past. However, contact marks of the claw marked on the roller appear on a full-color image. In order to eliminate the problem, a non-contact type separating plate is widely used in recent years. With the non-contact type separating plate, it is important to adjust a separation gap between the separating plate and the roller. Therefore, a steel metal plate is typically used as the non-contact type separating plate in order to achieve a required positional accuracy. Furthermore, such steel metal plate has not a claw-like shape but a plate-like shape so as not to cause image deterioration due to scratching onto the recording medium.
As described above, in order to achieve energy saving, the recent fixing device is required to be made into a stand-by state as fast as possible by rapidly heating the fixing device after an apparatus has been powered ON. Therefore, a configuration in which heat supplied to the pressing roller is suppressed to the minimum in order to heat the fixing roller much faster at the time of preparing for the stand-by state (rising) is employed. However, with such configuration, a difference in temperatures between the fixing roller and the pressing roller immediately after the rising is large. Accordingly, a recording medium immediately after passing through the fixing nip portion is largely back-curled to the pressing roller side due to the difference in temperatures between the front and back surfaces thereof.
In addition, in order to lower the heat amount of the pressing roller for speeding up the rising, an elastic layer on a surface of the pressing roller is required to be made as thin as possible. If the elastic layer on the surface of the pressing roller is made thinner, the pressing roller bites into the fixing roller so that an exit of the fixing nip portion directs to the pressing roller side. This causes a problem that a recording medium is easily wound up around the pressing roller.
Furthermore, when backing paper is used as a recording medium or duplex printing is performed, the following problem occurs. When a large amount of toner has been already adhered to a non-image surface of the recording medium, the recording medium is easily wound up around the pressing roller due to strong adhesive force between the non-image surface of the recording medium and the pressing roller.
Conventionally, in order to separate and convey a recording medium that is easily wound up around the fixing roller due to viscosity of toner, a separating mechanism is arranged on an image surface side and only a conveyance guide made of resin is arranged on a non-image surface side in a supporting manner in normal cases. However, for the reasons described above, in order to separate and convey the recording medium that is wound up around the pressing roller, a separating member having high positional accuracy is also required to be arranged on the non-image surface side. In this case, a non-contact type separating member is needed to be used not only on the image surface side but also on the non-image surface side for ensuring required image quality. Therefore, a separating plate is needed to be used on both the image surface side and the non-image surface side.
Conventionally, when jam of a recording medium occurs in a fixing device, a conveyance guide is configured so as to be opened or closed by a user or a service person for removing paper left on a fixing nip portion. However, when a separating plate is used on both the image surface side and the non-image surface side, a distance between both of the separating plates is normally narrow. Therefore, it is still difficult to make one's finger reach the fixing nip portion on a rear side of the separating plates even when the conveyance guide is opened. Furthermore, since the separating plates formed by a steel metal plate become high temperature exceedingly, it is dangerous for the user or the service person to touch the separating plates.